To obtain a better understanding of the contribution of the present invention to the state of the art in question, it should be observed that the rate at which a solution of phosphoric acid reacts with phosphate depends, amongst other things, on a good distribution of phosphate grains in the acid solution. The function of this reaction is to transform the phosphorous contained in the ore in a form that cannot be assimilated by plants into another form that can be assimilated.
Inserting phosphate directly into a reaction tank, i.e. into a reactor having a stirred reaction tank, would give rise:
firstly to a drop in the rate at which phosphate is converted because a portion of the phosphate would leave the reactor without reacting; and PA1 secondly to losses due to phosphate escaping in the waste gases. PA1 1) present improved granulation efficiency ensuring good physical quality to the finished product with a recycling rate close to three; PA1 2) only a small amount of dust is produced and it is suitable for proper disposal with anti-pollution standards being guaranteed in each case; PA1 3) only a small amount of energy is required to implement the method and the equipment required is simple, reliable, and compact; PA1 4) the method guarantees an increase in the capacity of units already in service in the range 30% to 100%; PA1 5) above all, the method makes it possible, by using appropriate equipments, to avoid the problems usually encountered in the slurry circuits, in particular blocking or clogging by the slurry setting, and also the presence of pockets of gas and pumping; PA1 6) the solution adopted enables starting and stopping to be performed rapidly while also making production easy to run; PA1 7) the method can be adapted to installations that are already in service; and PA1 8) the solution adopted is also remarkable in that it prevents return of the reaction gases. PA1 i. preparing a slurry of previously ground phosphate and a solution of phosphoric acid; PA1 ii. granulating the slurry; PA1 iii. drying the granulated product; PA1 iv. separating the granulated product by grain size; PA1 v. disposing of and scrubbing waste gases; and PA1 iv. recycling rejected finished product after grain size separation; PA1 wherein step i includes an operation of unclogging a premixer for the phosphate and the acid and situated upstream from a reactor having a reaction tank in which the acid solution attacks the granulated phosphate, which unclogging operation is performed by guided insertion of the previously ground and measured-out phosphate along a deformable axial path into the premixer and by spraying the phosphoric acid solution onto the phosphate at the end of the guided axial insertion path. PA1 measuring-out equipment for measuring out previously ground phosphate; PA1 a premixer for mixing the ground phosphate as measured out with a solution of phosphoric acid, the premixer having a cylindrical top portion running into a tapering bottom portion; PA1 a stirred-tank reactor receiving the phosphate and the phosphoric acid solution premixed in this way and intended to allow the phosphoric acid to attack the phosphate; PA1 a granulating drum for granulating the slurry leaving reactor; PA1 a drying device for drying the product granulated in this way; PA1 a plurality of separators for separating said granulated product by grain size; PA1 scrubbing devices for scrubbing the reaction gases, the drying gases, and the waste gases from the various apparatuses and the workshop; and PA1 a recycling circuit for recycling to the granulating drum rejected finished product after grain size separation; PA1 wherein the premixer includes unclogging means comprising in combination a flexible and resilient axial sleeve, in particular fixed to the top of the premixer by a collar and intended to receive the previously ground and measured-out phosphate, and at least one phosphoric acid solution sprayer injecting the acid solution under pressure level with the free bottom end of the sleeve, said sleeve including at least one slot giving access to the sprayed phosphoric acid solution and also constituting a non-return valve for reaction gases under the effect of the injected phosphoric acid and of its own resilience.
That is why a premixer is used to mix the granulated phosphate and the phosphoric acid solution prior to entering the reaction tank. However, although it is true that a conventional premixer enables the phosphate to be wetted, use thereof gives rise in turn to problems such as blocking or clogging of the premixer and to gases and vapors being given off thereby, thus interferring with operation and requiring frequent unclogging to be performed.
An object of the present invention is to eliminate these drawbacks of the methods and installations used in the prior art and it seeks to provide a method and an installation for producing granulated TSP that are substantially improved, that satisfy practical requirements better than previously known methods and installations, in particular in that: